Method of regulating and controlling speed and direction of electric motors and appliance

ABSTRACT

A control circuit for a reversible electric motor to be driven from an a.c. supply. Two thyristors are connected on respective sides of the motor in a series circuit including the thyristors, the motor and the a.c. terminals. The thyristors are inversely connected with respect to each other and a potentiometer is connected with respective ends to the thyristor trigger electrodes. A potential divider is connected across the supply terminals and there is a connection between the potential divider and the wiper on the potentiometer.

United States Patent [1 1 Chiles METHOD OF REGULATING AND CONTROLLINGSPEED AND DIRECTION OF ELECTRIC MOTORS AND APPLIANCE [75] inventor:Seaborn Howard Chiles, San

Francisco, Calif.

[73] Assignee: Chiles (Patents and Systems) Limited, Hong Kong, HongKong [22] Filed: Oct. 29, 1971 [21] Appl. No.: 193,912

[52] US. Cl. 318/257, 318/295 [51.] Int. Cl. H02p 5/16 [58] Field ofSearch 318/256, 257, 291,

[56] References Cited UNITED STATES PATENTS 3,214,667 10/1965 Foster318/345 3,593,077 7/1971 Loshbough ..3l8/293 [111 3,753,0 5 [451 Aug.14,1973

3,188,542 Dietrich 318/257 I 3,209,227 9/1965 Berman 318/257 3,249,8385/1966 .Mierendorf 318/293 3,568,024 3/1971 Robbins 318/293 PrimaryExaminer-Bernard A. Gilheany Assistant Examiner-Thomas LangerAttorney-E. F. -Wenderoth et al.

[5 7] ABSTRACT A control circuit for a reversible electric motor to bedriven from an ac supply. Two thyristors are connected on respectivesides of the motor in a series cir-- cuit'including the thyristors,the'motor and the a.c. terminals. The thyristors are inversely connectedwith respect to each other and a potentiometer is connected withrespective ends to the thyristor trigger electrodes. A potential divideris connected across the supply terminals and there is a connectionbetween the potential divider and the wiper on the potentiometer.

10 Claims, 3 Drawing Figures 1 METHOD OF REGULATING AND CONTROLLINGSPEED AND DIRECTION OF ELECTRIC MOTORS AND APPLIANCE The inventionrelates to a control circuit for a reversible electric motor, by whichis meant an electric motor whose direction of rotation is determined bythe direction of the supply current thereto.

According to the invention there is provided a control circuit for areversible electric motor, the circuit comprising: first and secondterminals for the motor; first and second terminals for an a.c. supply;a first thyristor connected with its anode/cathode path in seriesbetween the first supply terminal and the first motor terminal; a secondthyristor connected with its anode/- cathode path in series between thesecond supply terminal and the second motor terminal, the thyristorsbeing inversely connected with respect to the current path from thesupply terminal; a diode connected inversely across each thyristor; apotentiometer connected with respective ends to the trigger electrodesof the thyristors; a potential divider connected across the supplyterminals and a circuit connection between a tap on the potentialdivider and the wiper of the potentiometer.

Preferably there is a diode in series in the circuit connectionconnected with its cathode connected ot the wiper of the potentiometer.

Preferably there is provided an adjustable biasing resistor connectedbetween the cathode and the trigger electrode of each thyristor tocompensate for differences in S C R and motor characteristics.

The invention also includes within its scope a control circuit of thekind described above in combination with the electric motor to becontrolled connected between the two motor terminals. The motor, beingreversible, may have a permanent armature or field constituted by apermanent magnet or having a winding supplied from a separatedirect-current source. However, preferably the motor is a universalmotor, namely a motor with wound armature and field windings without aseparate direct-current supply. In order to make such a motor reversibleit is preferred to derive a unidirectional current for the field windingor the armature from the applied motor current which may be of eitherpolarity. To this end, it is preferred that the armature or fieldwinding be connected in a full-wave bridge rectifier circuit constitutedby four diodes included in the motor. The motor may be shunt wound orseries wound.

The invention will further be described with reference to theacompanying drawings, of which:

FIG. 1 is a circuit diagram of a control circuit according to theinvention in combination with an electric motor; and

FIGS. 2 and 3 are circuit diagrams showing alternative configurationsfor the motor.

Referrring to FIG. 1 the circuit is for controlling an electric motorconstituted by an armature A and a field winding F connected acrossmotor terminals M1 and M2. Current supply for the motor is derived froma supply of 120 volts. a.c. at 60 Hz connected across terminals PTl andPT 2. It will be appreciated that the voltage and frequency given areexemplary only and the invention is applicable to other supply and motorvoltages and frequencies. The speed of the motor is controlled by twothristors, or silicon controlled rectifiers SCRl and SCR2. ThyristorSCRl is connected with its anode to terminal PTl' and its cathode toterminal M1 whereas thyristor SCR2 is connected with its anode toterminal PM and its cathode to terminal M2. Diodes D2 and D3 areconnected inversely across thyristors SCRl and SCR2 respectively. Itwill be seen that for one direction of current flow a controlled currentis passed through thyristor SCRl, through the motor, and through diodeD3. For the other direction of current flow the current is controlled bythyristor SCR2, passes through the motor and through diode D2. ThyristorSCRl may conduct during positive half cycles of the supply (whenterminal PTl is positive with respect to terminal P12) and thyristorSCR2 may conduct during negative half-cycles of the supply.

The amount of current allowed to pass through the motor is controlled bythe conduction period in each half-cycle of the respective thyristor.This conduction time is controlled by the setting of a potentiometer R3,the ends of which are connected to the trigger electrodes of therespective thyristors. A potential divider is constituted by theresistors R1 and R2 connected across terminals PH and PT2. The tap onthe potential divider is connected through a circuit connectionincluding a diode D1 (a safety device used to prevent negative pulses atthe gate which could destroy the SCR) to the wiper of potentiometer R3.Resistors R1 and R2 are of suitable balancing values and with the wiperof the potentiometer at its central position the circuit is balanced andany current which flows through diode D1 is insufficient to trigger SCRlof SCR2. This is because of variable resistors R4 and R5 which areconnected respectively between the trigger electrode and the cathode ofthyristors SCRl and SCR2. These variable resistors are set so that withthe wiper of the potentiometer centered, both thyristors are biased off.Under these circumstances neither thyristor is triggered and no currentpasses to the motor. If, from this position, the wiper of thepotentiometer is moved towards the end connected to thyristor SCRI thenthe thyristor will be triggered to conduct in each positive half-cycleof the supply for a conduction period determined by the setting of thewiper. This will allow positive current to flow in pulses through themotor and the motor will turn in one direction. If the potentiometerwiper is now returned to the central position the motor will slow andstop. If the wiper is now moved in the other direction the thyristorSCR2 will be triggered to conduct during negative half-cycles forconduction periods dependent upon the setting of the potentiometerwiper. This will allow the motor to turn in the opposite direction.

The setting of the variable resistors R4 and R5 determines the biascondition for the thyristors and governs the point at which they startto conduct in each halfcycle. In this way the width of the flat spot" inthe centre of the potentiometer during which the motor does not turn canbe adjusted and also compensation can be made for differences in SCRsand motors.

With the circuit described there is a feedback effect whichautomatically regulates the speed of the motor against the effect ofincreasing or decreasing load. When the potentiometer is set to give apredetermined speed of rotation the motor will receive pulses of currentof a certain width or number of degrees of the sinewave in each halfcycle and will run at a corresponding speed. Should the load on themotor then be increased the motor will tend to slow. However, underthese conditions the back e.m.f. generated by the motor will fall off.This back e.m.f. tends to back-off the trigger potential applied throughdiode D1 since it opposes the driving potential across the potentialdivider R1, R2. Therefore, if the back e.m.f. falls off the effect is toincrease the thyristor conduction time in each half-cycle of the supplyand thereby increase the torque of the motor so that speed ismaintained. Conversely, if the load on the motor should decrease theresultant tendency for the back e.m.f. to increase will reduce thethyristor conduction time and the tendency for the motor speed to risewill be inhibited.

The motor shown in FIG. 1 is a universal shunt motor which has beenmodified. The modification consists in supplying the field winding Ffrom the output of a fullwave bridge rectifier constituted by diodes D4and D7. It will be seen that whatever the direction of current suppliedto the motor, the direction of current flowing through the field windingremains the same. This means that if the motor current is reversed thenonly the armature current will be reversed and the armature will turn inthe opposite direction.

Referring to FIG. 2 there is shown an alternative arrangement for themotor in which the armature, instead of the field winding is suppliedfrom the output of the bridge rectifier.

In FIG. 3 there is shown another alternative arrangement for the motorin which the armature and field winding are connected in series, thebridge rectifier being again connected to supply the field winding.However, it will be appreciated that the bridge rectifier mayalternatively be arranged to supply the armature in this configurationalso.

It will be seen that in the arrangements described above with referenceto the accompanying drawings there is no manually operated mechanicalswitch for reversing the motor.

There are many applications for a control circuit of the kind describedwith reference to the drawings. The

chines; hospital beds; hydraulic valves on bulldozers and earth movingmachines; chain hoists; cable hoists;

the motor; first and second terminals for an a.c. supply; a firstthyristor connected with its anode/cathode path in series between thefirst supply terminal and the first motor terminal; a second thyristorconnected with its anode/cathode path in series between the secondsupply terminal and the second motor terminal, the thyristors beinginversely connected with respect to the current path from the supplyterminal; a diode connected inversely across each thristor; apotentiometer connected with respective ends to the trigger electrodesof the thyristors; a potential divider connected across the supplyterminals and a circuit connection between a tap on the potentialdivider and the wiper of the potentiometer.

2. A control circuit as claimed in claim 1 wherein there is a diode inseries in the circuit connection connected with its cathode connected tothe wiper of the potentiometer.

3. A control circuit as claimed in claim 1 wherei there is provided anadjustable biasing resistor connected between the cathode and thetrigger electrode of each thyristor.

4. The combination of a control circuit as claimed in claim 1 with anelectric motor connected between the motor terminals.

5. The combination claimed in claim 4 wherein the motor is a universaltype motor and means are provided for supplying the field windingthereof with unidirectional current of either polarity.

6. The combination claimed in claim 5 wherein the means for supplyingthe field winding comprises a rectifier arrangement for deriving theunidirectional current from the current supplied to the motor terminals.

7. The combination as claimed in claim 6 wherein the field winding isconnected in a full-wave bridge rectifier circuit consituted by fourdiodes included in the motor.

8. The combination claimed in claim 4 wherein the motor is a universaltype motor and means are provided for supplying the armature windingthereof with unidirectional current of either polarity.

9. The combination claimed in claim 8 wherein the means for supplyingthe armature winding comprises a rectifier arrangement for deriving theunidirectional current from the current supplied to the motor terminals.

10. The combination as claimed in claim 9 wherein the armature isconnected in a full-wave bridge rectifier circuit constituted by fourdiodes included in the motor.

1. A control circuit for a reversible electric motor, the circuit comprising: first and second terminals for the motor; first and second terminals for an a.c. supply; a first thyristor connected with its anode/cathode path in series between the first supply terminal and the first motor terminal; a second thyristor connected with its anode/cathode path in series between the second supply terminal and the second motor terminal, the thyristors being inversely connected with respect to the current path from the supply terminal; a diode connected inversely across each thristor; a potentiometer connected with respective ends to the trigger electrodes of the thyristors; a potential divider connected across the supply terminals and a circuit connection between a tap on the potential divider and the wiper of the potentiometer.
 2. A control circuit as claimed in claim 1 wherein there is a diode in series in the circuit connection connected with its cathode connected to the wiper of the potentiometer.
 3. A control circuit as claimed in claim 1 wherein there is provided an adjustable biasing resistor connected between the cathode and the trigger electrode of each thyristor.
 4. The combination of a control circuit as claimed in claim 1 with an electric motor connected between the motor terminals.
 5. The combination claimed in claim 4 wherein the motor is a universal type motor and Means are provided for supplying the field winding thereof with unidirectional current of either polarity.
 6. The combination claimed in claim 5 wherein the means for supplying the field winding comprises a rectifier arrangement for deriving the unidirectional current from the current supplied to the motor terminals.
 7. The combination as claimed in claim 6 wherein the field winding is connected in a full-wave bridge rectifier circuit consituted by four diodes included in the motor.
 8. The combination claimed in claim 4 wherein the motor is a universal type motor and means are provided for supplying the armature winding thereof with unidirectional current of either polarity.
 9. The combination claimed in claim 8 wherein the means for supplying the armature winding comprises a rectifier arrangement for deriving the unidirectional current from the current supplied to the motor terminals.
 10. The combination as claimed in claim 9 wherein the armature is connected in a full-wave bridge rectifier circuit constituted by four diodes included in the motor. 